1. Field of the Invention
The present invention relates to a gas diffusion electrode for use in a solid polymer electrolyte type fuel cell, a method for manufacturing the same, and a fuel cell with such an electrode.
2. Description of the Related Art
A solid polymer electrolyte type fuel cell is an electrochemical apparatus in which, for example, hydrogen is supplied as fuel to an anode and oxygen is supplied as oxidant to a cathode so that electric power is obtained by electrochemical reaction between the hydrogen and oxygen. The anode and cathode are gas diffusion electrodes. The anode is joined to one surface of an electrolyte membrane while the cathode is joined to the other surface of the same so that a gas diffusion-electrode/electrolyte-membrane assembly is formed.
Each gas diffusion electrode is constituted by a gas diffusion layer and a catalyst layer. The catalyst layer of each of the anode and cathode is provided with metal particles of the platinum group metal, carbon particles supporting such metal particles, or the like, as catalyst. Porous carbon paper, or the like, having hydrophobicity, is used for the gas diffusion layer. An single cell as a basic unit is formed by a structure in which such a gas diffusion-electrode/electrolyte-membrane assembly is held between a pair of gas impermeable separators provided with gas feed channels. A solid polymer electrolyte type fuel cell is formed by stack of a plurality of such single cells.
When a solid polymer electrolyte type fuel cell is operated, the following electrochemical reactions proceed.
anode: 2H2xe2x86x924H++4exe2x88x92
cathode: O2+4H++4exe2x88x92xe2x86x922H2O
Generally, in such a fuel cell in which oxygen and hydrogen are made to react with each other, the fact that activation overvoltage in the oxygen reductive reaction is high is one of the reasons for lowering voltage in high current density. Therefore, a catalyst such as platinum group metal or the like is added to each electrode in order to reduce the activation overvoltage.
In such a solid polymer electrolyte type fuel cell having a solid polymer electrolyte membrane, the electrochemical reactions in the anode and cathode proceed in so-called three-phase interfaces formed by the catalyst, the electrolyte, and reactants which are included in the cathode and anode. Therefore, in order to obtain high power in such a fuel cell, it is requested to increase the contact area between the catalyst and the electrolyte.
Methods in which irregularities are provided on the surface of an electrolyte membrane so as to increase the contact area between each electrode including the catalyst and the electrolyte membrane, particularly between a catalyst layer in the electrode and the electrolyte membrane have been devised in order to obtain high power in a solid polymer electrolyte type fuel cell. One of those methods is a method in which the surface area of a solid polymer electrolyte membrane is increased so as to increase the contact area between the electrolyte membrane and each electrode. There has been proposed a method for giving irregularities onto the surface of a solid polymer electrolyte membrane, for example, by such a method using a roll with irregularities as disclosed in JP-A-3-158486, by such a method using sputtering as disclosed in JP-A-4-169069, by such a method using plasma etching as disclosed in JP-A-4-220957, or by such a method of embedding cloth and then peeling the embedded cloth as disclosed in JP-A-6-279600.
There is another method in which pores are provided on the surface of a solid polymer electrolyte membrane so as to increase the contact area between the electrolyte membrane and a catalyst layer. For example, JP-A-58-7432 discloses a method in which a dispersion medium dissolving an electrolyte is crystallized into droplets and then these droplets are removed from the electrolyte, JP-A-62-146926 discloses a method in which particles are embedded in an electrolyte and then removed from the electrolyte, and JP-A-5-194764 discloses a method in which organic material is mixed with an electrolyte and then removed from the electrolyte.
There is a further method in which platinum group metal is supported on the surface of an electrolyte membrane so as to increase the contact interface between the electrolyte and a catalyst. For example, JP-B-59-42078 or JP-B-2-43830 discloses a method in which electroless plating is given to the surface of an electrolyte. Further, there is a method in which an electrolyte is added to a catalyst layer so as to increase the contact area between the catalyst and the electrolyte. For example, JP-B-2-7398 discloses a method in which an electrode is produced out of a mixture of an electrolyte solution and fluorocarbon resin such as PTFE or the like, and JP-B-2-7399 discloses a method in which an electrode is produced out of a catalyst coated with an electrolyte and fluorocarbon resin such as PTFE or the like. In addition, U.S. Pat. No. 5,211,984 discloses a method in which an electrode is produced out of a mixture of a catalyst and an electrolyte solution.
In such a method using a roll, such a method using sputtering, such a method using plasma etching and such a method using cloth, as described previously, there was a problem that the processing for providing irregularities was so troublesome that the productivity deteriorates, or the formed irregularities are too rough to increase enough the contact area of the interface between an electrolyte membrane and each electrode.
In a method in which pores are formed by removing a crystallized dispersion medium, planted particles or mixed organic material, it is difficult to completely remove the dispersion medium, particles or organic material. Such a residue becomes an obstacle to the contact between an electrolyte membrane and each electrode or to ion conduction between the electrodes. Heating treatment or solvent treatment performed in a process of removing such a residue causes deterioration in the ion conductivity. For these reasons, in a fuel cell manufactured by use of an electrolyte membrane in which the contact area was increased by the conventional method, there was a problem that the improvement of the performance was not sufficient.
Platinum group metal formed on the surface of an electrolyte membrane by a method using electroless plating or the like was low in activity as a fuel cell catalyst due to its small surface area, so that the improvement of the catalyst activity was not sufficient in this method.
J. Electrochem. Soc., 140, 3513 (1993) indicates a problem in a catalyst layer consisting of a catalyst and an electrolyte. That is, in such a catalyst layer, the resistance to proton conduct is large, and portions which are not much affected by the lowering of voltage due to the resistance to proton conduct are concentrated in the vicinity of the electrolyte membrane. If the catalyst quantity of the catalyst layer is increased, the thickness of the catalyst layer is also increased. As a result, the influence of the lowering of voltage due to the resistance to proton conduct of the catalyst layer becomes strong in high current density. Therefore, the characteristics of the catalyst layer cannot be improved simply only by increasing the catalyst quantity.
It is an object of the present invention to form a large number of so-called three-phase interfaces where electrode reactions proceed in accordance with the increase of the catalyst quantity of a catalyst layer so as to reduce activation overvoltage. It is another object of the present invention to improve the proton conductivity of the catalyst layer so as to reduce the lowering of voltage due to the resistance to proton shift. It is another object of the present invention to form appropriate pores in the catalyst layer so as to improve the property of feeding reactants to the three-phase interfaces to thereby reduce the concentration overvoltage. It is a further object of the present invention to provide a high-power fuel cell through these improvements.
A fuel cell gas diffusion electrode according to the present invention has a catalyst layer and a gas diffusion layer. The catalyst layer is produced in the following manner. A porous electrolyte A with pores three-dimensionally communicating with one another is formed, and then a micro-porous catalyst-electrolyte aggregate including a catalyst and an electrolyte B is provided in the pores of the porous electrolyte A. This porous electrolyte A is produced by a method capable of preventing lowering of the ion conductivity due to the mixing of impurities or deterioration.
Perfluorosulfonic resin having proton conductivity is used as electrolyte in the solid polymer electrolyte type fuel cell. Generally, this electrolyte has a characteristic that the proton conductivity is high if the ion exchange capacity is high, and on the other hand, the solubility of reactants such as oxygen or the like is high if the ion exchange capacity is low. With this characteristic, the proton conductivity of the electrolyte contained in the catalyst layer and the property of feeding the reactant are improved separately in accordance with the respective functions. As a result, the proton conductivity of the catalyst layer as a whole and the property of feeding the reactant are improved.
That is, the porous electrolyte A with pores three-dimensionally communicating with one another is formed in the catalyst layer out of an electrolyte which is relatively large in the ion exchange capacity and high in the proton conductivity. Thus, the proton conductivity is improved. A catalyst coated with the electrolyte B which is relatively small in the ion exchange capacity and large in the solubility of the reactant is provided in the pores of the porous electrolyte A. Thus, the property of feeding the reactant to the three-phase interfaces is improved.
According to a first aspect of the present invention, there is provided a gas diffusion electrode comprising a catalyst layer and a gas diffusion layer, wherein the catalyst layer has a porous electrolyte A with pores which three-dimensionally communicate with one another, and a micro-porous catalyst-electrolyte aggregate including a catalyst and an electrolyte B is provided in the pores.
According to a second aspect of the present invention, there is provided a method for manufacturing a gas diffusion electrode, in which a micro-porous catalyst-electrolyte aggregate including a catalyst and an electrolyte B is provided in pores of a porous electrolyte A which three-dimensionally communicate with one another.
According to a third aspect of the present invention, there is provided a gas diffusion electrode comprising a catalyst layer and a gas diffusion layer, wherein the catalyst layer has a structure in which a porous electrolyte A with pores three-dimensionally communicating with one another is provided, and a micro-porous catalyst-electrolyte aggregate including a catalyst and an electrolyte B is provided in the pores, the porous electrolyte A having an ion exchange capacity which is larger than an ion exchange capacity of the electrolyte B in the pores of the porous electrolyte A.
According to a fourth aspect of the present invention, there is provided a gas diffusion electrode of the first or third aspect, wherein the porous electrolyte A and said electrolyte B in said pores of said porous electrolyte A comprises perfluorosulfonic resin.
According to a fifth aspect of the present invention, there is provided a solid polymer electrolyte type fuel cell comprising a gas diffusion electrode of the first, third or fourth aspect.